The present invention relates to dosimeters for evaluating the degree of illumination of objects, such as woks of art, during their display, in particular in museums and temporary exhibitions.
Some cultural heritage objects are very fragile and their exposure to light has to be restricted to a few tens of thousands of lux.hour, in conformity with very strict recommendations.
Monitoring measurements can be taken using photoelectric cells but this selective evaluation quickly reaches its limits during a loan or when the items are subjected to natural lighting. Thus, it often proves difficult to recognize the light dose received by the items and to know whether the prescribed limit has been exceeded.
Several attempts have been made to develop a dosimeter by impregnating a paper substrate, a polymer or a glass substrate with an organic dye. Indicators of exposure (illuminancexc3x97time), such as Rhodamine B, provided in the form of strips of filter paper impregnated with a solution of Rhodamine B, and Litharge, as a paste applied to a paper cut into a strip and dried in darkness, are known in particular. Toshiko Kenjo, xe2x80x9cCertain deterioration factors for works of art and simple devices to monitor themxe2x80x9d, The International Journal of Museum Management and Curatorship (1986), 5, 295-300. These suggestions have not, to the knowledge of the Applicant, resulted in the production of a tool used or usable in museums and collections.
Currently, the only solution widely employed in institutions consists in using scales of eight samples of wool dyed with blue dyes of increasing stability toward light. These scales were developed for the textile industry in order to determine the index of stability toward light of textile dyes.
These scales are exposed close to the items and the damage experienced by the blue scale is estimated visually or using a calorimeter. This method has limits, these scales are not very sensitive and it is difficult to quantitatively evaluate the extent of the irradiation. If precise measurements are desired, it is necessary to be equipped with an expensive measuring device, such as a calorimeter or a densitometer, in order to evaluate the degree of decomposition of the dyes, or else a cell which is permanently positioned close to the item and which measures the cumulative rumination. This device is expensive and bulky.
The invention is aimed at overcoming the disadvantages of the prior devices by creating a dosimeter which combines simplicity of design with a sensitivity suited to the exposure acceptable for fragile objects and with good reproducibility of the results.
A subject matter of the invention is thus a dosimeter for evaluating the degree of illumination by a change in coloration comprising a substrate on which is positioned a coloring substance which is sensitive to visible light, characterized in that the coloring substance is formed of a mixture of several organic dyes, which absorb in different spectral regions, dispersed in a polymer matrix and coated onto paper.
According to other characteristics of the invention:
the dyes are selected from the following dyes of the Color Index: CI: 42025, 42040, 42051, 42555, 42595, 42780, 44045, 45160, 45165, 45170, 45350, 45380, 4540 0, 45430, 5440, 46025, 49005, 49010, 52015 and 52040;
the polymer matrix comprises a binder based on vinyl acetate homopolymers which are sold as an aqueous emulsion by Raoul Labord, Z1, Chemin des 50 Arpents, Saint-Germain-lxc3xa8s-Arpajon;
BP.25-91291 arpajon cedex, under the trade name Savatex N 385;
the emulsion employed comprises, inter alia, triacetin and, as plasticizer, phthalates;
the mixture used can comprise a sensitizer, such as linoleic acid or oleic acid, with a concentration of 0.1 to a few percent;
the range of concentrations of the dyes is between 0.01% and 0.05%, according to the dyes and the required sensitivity.
A number of examples of mixtures of dyes used for the preparation of dosimeters according to the invention will now be described.